Lignocellulosic biomass is an abundant renewable resource and a potential feedstock for the production of liquid fuels and other value-added products (1). The principal barriers to the production of lignocellulose-derived biofuels are the high costs of chemical pretreatment and enzymes for depolymerization (2). The thermophilic fungus, Sporotrichum thermophile, very rapidly degrades cellulose, and metabolizes powdered cellulose and glucose at nearly same rates (3). The thermostability of the hydrolytic enzymes from this organism provides practical advantages, such as high enzymatic activity over a broad pH range, over those from the mesophilic fungus, Hypocrea jecorina (syn. Trichoderma reesei), which has traditionally been used for production of biomass degrading enzymes. During growth on cellulosic substrates, S. thermophile secretes cellulases, hemicellulases (4), oxidative enzymes (5), and many proteins of unknown function.
Cellobiose dehydrogenase (CDH) is an extracellular hemo-flavoprotein that is produced in large amounts by S. thermophile during growth on cellulose (5). CDH is produced by many cellulolytic fungi (6). It oxidizes the reducing end of cellobiose and longer cellodextrins to the corresponding aldonolactones (FIG. 1). For all cellulolytic microorganisms, the sugar acid yield from cellulose could be improved by increasing the expression level of CDH and glucose oxidases.
Aldonolactones, or sugar lactones, are unstable in aqueous solution and undergo hydrolysis to form the corresponding aldonic acids. The extent and rate of uncatalyzed hydrolysis are dependent on the specific lactone, pH, and temperature. The equilibrium constant between glucono-δ-lactone and gluconic acid is 7.7, favoring gluconic acid, and the half-life of glucono-δ-lactone in water at room temperature and pH 5.0 is approximately 1 hour (10). However, despite this lack of stability, fungi such as S. thermophile have evolved enzymes to catalyze the hydrolysis of sugar lactones to their corresponding aldonic acids. This hydrolysis to aldonic acid increases susceptibility of cellulose to subsequent hydrolysis by cellulolytic enzymes such as cellulases. Therefore, efficient conversion of sugar lactones to aldonic acids can have beneficial effects on cellulose degradation and, thus, on biofuel formation (FIG. 1).
Enzymatic hydrolysis of sugar lactones has been mostly studied in the context of the pentose phosphate pathway (11-14). In the pentose phosphate pathway glucose-6-phosphate is converted to 6-phospho-gluconolactone by glucose-6-phosphate dehydrogenase. The lactone is then hydrolyzed by 6-phosphogluconlactonase (PGL) to generate 6-phosphogluconate and finally converted to ribulose-5-phosphate. In glycolysis-deficient strains of Escherichia coli, deletion of the PGL gene leads to severe inhibition of growth on glucose (15), clearly demonstrating that spontaneous hydrolysis of 6-phosphogluconolactone is insufficient in vivo.
There have been reports of aldonolactonase activity secreted into the culture filtrates of diverse fungi (16-17). An aldonolactonase from Aspergillus niger was purified by Bruchmann et al. (16) and was shown to be an important part of the fungal cellulolytic system. However, extracellular aldonolactonases have not been purified, identified, or characterized from S. thermophile. 
Reactions at high temperatures and acidic pH values are critical for the enzymatic conversion of plant cell wall polysaccharides to fermentable sugars in the emerging biofuel industry. High temperature conversions lower the risk of bacterial contamination and enzymes usually have faster turnover at high temperatures. Low pH is also beneficial because of the reduced risk of contamination. Cellulases work optimally at pH 4.8-5.0 and it makes the process easier if all the enzymes have similar pH optima so they can be used simultaneously. Thus, it is important in the art to have thermostable enzymes isolated from thermophilic fungi, such as S. thermophile, that are active over a broad range of pH values for the hydrolysis of lactones produced during the conversion of biofuel feedstock plant cell wall polysaccharides to fermentable sugars. Compositions and methods comprising aldonolactonases active over a broad range of pH values will find utility in the enzymatic depolymerization of lignocellulose.